The secondary vitreous of the adult, which seems nonessential in its normal state, can be a highly significant structure when diseased, demonstrating its importance in health and disease. Vitreous pathology constitutes a major barrier to the successful treatment of vitreoretinal disorders. The vitreous has several significant functions, including the molecular sieve effect, prevention of vasoproliferation, and modification of inflammatory and immunological processes, as shown by recent experimental investigaions. However, our understanding of the pathophysiology of vitreoretinal disease in human is incomplete; only a few chemical, morphological, and immunological studies have been done on pathological human vitreous. The major reason for this is the scarcity of available vitreous specimens. The advent of new vitrectomy procedures provides the unique opportunity to investigate previously discarded human pathological material, which can be obtained in situ during the surgery in an unaltered state. This investigation will study pathological vitreous obtained in situ from human eyes undergoing open-sky as well as closed vitrectomy. It will focus on the clinico-pathological correlation of the vitrectomy material, by use of ophthalmological, chemical, biochemical, tissue culture, immunological, and morphological methods. We will analyze macromolecular composition of pathological vitreous, possible degradation of hyaluronic acid, and regenerative properties of human vitreous. The morphology of pathological vitreous membranes and preretinal membraes will be studied, in order to determine which cells proliferate and produce the membranes. Tissue culture techniques will be used to determine biosynthetic properties of the pathological cellular membranes, synthesis of collagen, and presence of growth-promoting factors. Immunological methods will be applied to study the immunological effect of normal and pathological human vitreous on the stimulation of lymphocytes under tissue culture conditions. All experimental and clinical data will be correlated to the particlar vitreoretinal disease, the postoperative follow-up, and the surgical outcome.